The Authors analyse the histopathological alterations observed in 378 cases of Mucocutaneous Leishmaniasis from Três Braços, Bahia, of whom 307 (81,4%) were only cutaneous, 54 (14,2%) only mucosae and in 17 (4,4%) both integuments were involved simultaneously. A cellular infiltration of lymphocytes and plasma cells was invariably present in all forms and during healing. In mucosal cases, this reaction may have an important role in maintaining an auto agression in the presence of few or no parasites. The plasma cell is a constant ele-ment in well developed lesions but not present in healing lesions. Its presence almost always denotes a tendancy to relapse. Mast cells were observed in both cutaneous and mucosal lesions but predominate in the former. Its number was significantly greater in those with an exudative and necrotic granulomatous reaction where necrotic phenomena are marked. Eosinophils were significantly associated with mast cells suggesting the existence of association between these cells in the modulation of the inflammatory response. Two types of granulomatous reaction were observed: a disorganized one, often related to tissue necrosis, and an organized, tuberculoid type granuloma. The first type is interpreted as of post necrotic origin resulting in a reduction in antigenic load with maintenance of antibody levels, creating the conditions to establish a granuloma similar to that observed in immunocomplex lesions with excess of antibody. The other type of rection was the granuloma of epithelioid cells, which tend to arise in two groups of patients. In young patients, with short time cutaneous lesions and normal positive leishmanin skin tests, this type of granuloma perhaps similar to the specific hypersensibility granuloma described by EPSTEIN (1977). In the other group of patients, the leishmanin skin test was always exaggerated. In these cases the hypersensitive granuloma is associated with a cellular hypersensivity, perhaps amplified by sequestration of antigen, reforcing the antigenic stimulus of the granuloma. As a result, treat- ment of this group is more difficult and the prognosis worse in consequence.